You must use the same name and location, when you save-as a .png
Photoshop is wonderful at changing your file names and your directory causing you to navigate to where you really want it to be saved. So Vegas is looking at the old graphic, not working from memory. If you want to be sure, in the project window, select your graphic and explore current directory. Check the Date-modified to see if it is the last time you saved the file.
The script Mike Kujbida recommends is good if your source is a PSD. Why not use the editable versions?
Jeff
http://www.narrowroadmedia.com